This is an issue some users have reported when reopening ebooks they have previously downloaded and opened without issue. After trying to open the ebook this error message is displayed:

The developers are aware of this issue. They are investigating it at the moment and once a fix is developed it will be released in a future app update.

Unfortunately until it's fixed, the only solution is to uninstall the app and then reinstall it.

If you uninstall the app and reinstall it and still experience the issue, please take a screenshot of the error and send it to us by completing our support form to submit a ticket to our customer support. Please state that you have already tried the solution in this help article. Please also state which version of the app you have installed (iOS or Android), and your operating system's version number, and which ebook is experiencing the issue.